The invention relates to a drilling device for producing drill holes with an undercut and comprising a drill bit attached to a drill bit holder and a suction bell.
German Offenlegungsschrift 33 40 090 discloses a drilling device of the type mentioned in which a drill bit can be screwed into a drill bit holder. A suction bell is provided in the region of the drill bit holder. The suction bell surrounds the drill bit holder and the drill bit stem. The suction bell is attached with a suction hose to a suction fan of a drilling machine or similar apparatus. The purpose of the suction device is to suck off the drilling dust accumulating in the drill hole during the drilling operation. In this known device, the drill bit stem is guided with a lateral play in a suction sleeve so that the drill bit is pivotable within the suction sleeve. The pivotal movement enables to produce the undercut at the bottom of the drill hole.
Metal expansible fixing plugs can be inserted in a drill hole with an undercut. They expand in the region of the undercut and are, thus, anchored positively in the drill hole.
To produce an undercut at the bottom of the drill hole that is as accurate as possible, the pivotal movement of the drill bit has to be limited. This can be achieved by providing in the middle of the drill bit stem a cross-sectional enlargement which serves as a pivot bearing that engages the wall of the drill hole. The cross-sectional enlargement, however, makes it difficult to suck off the drilling dust, since there is only a small gap left clear in the region of the cross-sectional enlargement for sucking the drilling dust away.